The invention relates to methods and devices for transforming first frame data.
Over the years, frame formats to be encoded have become increasingly large due, for example, to the introduction of new types of recording systems, including the current changeover from a PAL (Phase Alternating Line) TV transmission system used in Europe over the last 50 years, having a 625×576 pixel frame size, to a 1920×1080 or 1280×720 pixel HDTV (High Definition Television) resolution. In the future, even larger frame formats are expected to be introduced in new types of TV systems.
HDTV and future systems use digital compression methods to compress a sequence of video frames so that they can be transmitted over the Internet or mobile communications channels for example. However, due to the increased frame format sizes, the computing power required for compressing the sequence of video data and the amount of memory required here are increasing significantly. One consequence of this is that data transfer between memory and processing units which implement the compression methods is also increasing significantly.
Therefore study groups such as the Joint Collaborative Team on Video Coding (JCT-VC), a common working party of the ITU and ISO/IEC (ITU—International Telecommunication Union, ISO—International Standardization Organisation, IEC—International Electrotechnical Commission) are working not only on improving the compression rate, but also on standardized methods for enabling video frames to be efficiently stored in reference frame buffers of the respective codec and accessed in a resource-saving manner.
FIG. 1 shows a known device for compressing a sequence of frames, comprising a reference frame buffer SRB. Frames are encoded, for example, using predictive coding, also know as inter-frame coding. One of the frames is divided into frame blocks BB, e.g. of 16×16 pixels, and is then encoded frame block by frame block. Then for one of the frame blocks a reference frame block RBB providing a good basis for estimating the content of the frame block is searched for in a reference frame REF. For this purpose the frame block is transferred to a motion estimation unit ME which, on the basis of a reference sub-frame REFT comprising parts of the reference frame REF after frame decompression by a frame decompression unit PD, selects the reference frame block from the reference sub-frame and signals the selected reference frame block to a motion compensation unit MC by a motion vector MV. The motion compensation unit provides the reference frame block on the basis of the reference frame and motion vector.
In a next step, a difference frame block BD is generated by subtracting the reference frame block RBB from the frame block BB. The difference frame block subsequently undergoes transformation in a transformation unit T, e.g. using a discrete cosine transform method. Available at the output of the transformation unit are transform coefficients TK which are then fed to a quantization unit Q for quantization. Available at the output of the quantization unit are quantized transform coefficients TQ which are converted into an output signal AS by entropy encoding performed by an entropy encoding unit EC.
In a feedback loop, the quantized transform coefficients TQ are converted into reconstructed transform coefficients TKR by inverse quantization by an inverse quantization unit IQ. These reconstructed transform coefficients TKR are transformed into a reconstructed difference frame block BDR by inverse transformation by an inverse transformation unit IT. In a further step, a reconstructed frame block RBM is generated by adding the reconstructed difference frame block BDR and the reference frame block RBB.
In older encoding methods, the reconstructed frame block is written directly to the reference frame buffer. In methods currently in standardization, to reduce a data volume the reconstructed frame block first undergoes frame compression by a frame compression unit PC which significantly reduces the data volume of the reconstructed frame block. A compressed reconstructed frame block RBC produced by the frame compression unit PC is then stored in the reference frame buffer. In order to enable the motion estimation unit and the motion compensation unit to access the required frame data, when a reference frame REF or rather a specific section of the reference frame is requested, the respective compressed reconstructed frame block is read out from the reference frame buffer SRB and converted into a reference sub-frame REFT by frame decompression by a frame decompression unit PD.
FIG. 2 shows a decoder corresponding to the encoder shown in FIG. 1. The output signal AS is decoded into quantized transform coefficients TQ by an entropy decoding unit ED. In addition, the quantized transform coefficients are inversely quantized into reconstructed transform coefficients TKR by the inverse transformation unit IQ. This is followed by inverse transformation of the reconstructed transform coefficients TKR into a reconstructed difference frame block BDR by the inverse information unit IT. In addition to the output signal, the respective motion vector MV, among other things, is also transmitted to the decoder. Using the reference sub-frame REFT, the decoder can determine therefrom, by the motion compensation unit MC, the reference frame block RBB which is converted into the reconstructed frame block RBM by adding it to the reconstructed difference frame block.
The reconstructed frame block RBM can be reproduced on a display, for example. The reconstructed frame block RBM is then converted by compression by the frame compression unit PC into the compressed reconstructed frame block RBC which is then stored in the reference frame buffer SRB. The compressed reconstructed frame blocks stored in the reference frame buffer can be decompressed into the reference sub-frame by the frame decompression unit PD.
The article by Chong Soon Lim (“Reference Frame Compression using Image Coder,” 2010) describes a lossless frame compression/decompression method in which bit-plane coding is carried out after floating point discrete cosine transformation (DCT) and scanning of coefficients in a one-dimensional representation, arranged two-dimensionally after the transformation.
In a method according to Mehmet Umut Demircin et al. (“Compressed Reference Frame Buffers [CRFB],” 2010), a buffer access bandwidth reduction technique is proposed. In addition to transformation and quantization, DC prediction and entropy encoding for the frame compression unit PC and/or a reverse step for the frame decompression unit PD are also proposed.
Madhukar Budagavi (ALF Memory Compression and IBDI/ALF coding efficiency test results in TMuC-0.1,” 2010) describes test results for compression and decompression of frame data upstream and downstream respectively of a deblocking frame memory.
Lastly, Hirofumi Aoki (“DPCM-Based Memory Compression,” 2010) describes a one-dimensional DPCM-based frame memory compression method (DPCM—discrete pulse code modulation).
At least the compression methods proposed by Lim and Aoki are lossless.